1. Field of the Invention
This invention relates to a blowing head for making tubular films from thermoplastic materials, wherein a lateral inlet for supplying molten material opens into a first annular passage having an upper boundary wall, which extends in a plane which is substantially radial with respect to the blowing head, and wherein a second annular passage is provided, which is concentric to the first annular passage and leads to a die orifice, from which the tubular film is extruded, said second annular passage is separated from the first annular passage by a tubular partition, which has a flow guiding surface, which adjoins both annular passages, and the axial length of said tubular insert, measured from the upper boundary wall to the flow guiding surface, decreases from the inner end of the inlet to the diametrically opposite line of confluence of the streams of molten material diverging from the inlet in such a manner that all flow paths from the molten material flowing from the inlet to the die orifice are substantially equal in length.
2. Description of the Prior Art
The output of plants from making blown tubular plastic films is substantially restricted by the cooling capacity because at least the speed at which the tubular film is taken off must be so low that at the time at which the tubular films are collapsed to a flat shape they are sufficiently cooled to ensure that the superimposed layers of film will not stick together. It has been found that air is an ideal fluid for cooling the extruded tubular films because an abrupt cooling, which would adversely affect the PG,3 quality of the film, will be avoided by the use of air. To ensure that the inflated tubular film will be cooled between the film blowing head and the collapsing device, the film blowing heads used in recent years inflate the tubular film by means of cooling air for internally cooling the inflated tubular film. The output rate which can be achieved with such blowing heads is vitually twice the output rate which can be achieved with tubular film blowing plants in which the inflated tubular film is air-cooled only on the outside. But the need for a replacement of the cooling and inflating air gives rise to the problem that the passages for supplying the air for internal cooling must extend through the film blowing head and must be so arranged that they will not adversely affect the conditions in the passages for supplying the molten material. Because the air for inflating and internally cooling the tubular film must be supplied at a high rate, said air for internal cooling can be supplied and withdrawn only in an axial direction. For this reason the molten material must be supplied to the film blowing head in a lateral direction unless a more expensive design is chosen, such as is apparent from German Patent Specification 23, 06 834.
Published German Application 20 09 914 discloses a film blowing head having a lateral inlet for molten material. A disadvantage of that known blowing head resides in that it is heated to different temperatures because the paths along which the molten material flows to the die orifice differ in length so that the temperature varies around the periphery of the film blowing head and it may be necessary to restore the die orifice to a centered position whenever the output rate or the plastic material is changed in such a manner that the temperature of the molten material will be altered.
In the film blowing head of the kind described first hereinbefore and known from German Patent Publication 17 29 055, the disadvantage residing in the presence of flow paths of different lengths for the molten material from the inlet to the die orifice from which the tubular film is extruded is avoided in that the molten material is deflected near the inlet to flow a way from the die orifice, the deflected molten material is subsequently deflected around a flow guiding surface having an approximately semielliptical shape to flow toward the die orifice, and the molten material is then guided to the die orifice. But it has been found that the tubular films made with that known film blowing head have a thin portion corresponding to the line of confluence of the streams of molten material that have diverged near the inlet.